The Contribution of Grain-Boundary Diffusion to Creep at Low Stresses

Abstract

Experiments are described on the deformation of copper, both in the form of long straight wires and in the form of helices, with the aim of evaluating the importance of diffusion-creep processes, especially in the range where the effect of grain-boundary diffusion is expected to predominate. It is established that at low stresses and elevated temperatures, polycrystalline copper can behave in a Newtonian manner with the primary creep stage of negligible importance. Above a specific temperature, which is shown to vary inversely as the logarithm of the grain size, creep rates agree closely with the Nabarro–Herring equation, and the activation energy is identical with that for lattice self-diffusion. Below this temperature the activation energy is found to be near the value expected for grain-boundary self-diffusion, and the creep rate now varies inversely as the cube of the grain size in agreement with the equation derived by Coble (J. Applied Physics, 1963, 34, 1679). Within the range of the present experiments, there was no evidence of a contribution to creep by a mechanism of dislocation-pipe diffusion and grain boundaries were the only sources and sinks for vacancies.